The present invention relates in general to a device for measuring the oxidizable carbon of a liquid, and more particularly, to a device for determining the oxidizable carbon content of a liquid by obtaining an accurate thermally-corrected conductivity measurement of a photo-oxidized sample of said liquid.
Instruments (and methods) for photo-oxidizing water and measuring the total oxidizable carbon (TOC) content thereof are well-known. Such devices operate primarily by exposing a sample of water, presumably containing dissolved organic constituents, to ultraviolet radiation (xe2x80x9cUVxe2x80x9d), while contemporaneously measuring the change in electrical conductivity of the sample due to the formation of carbon dioxide by oxidation of said organic constituents. See e.g., U.S. Pat. No. 4,626,413, issued to F. K. Blades et al. on Dec. 2, 1996; 4,666,860, issued to F. K. Blades et al. on May 19, 1987; and U.S. Pat. No. 5,047,212, issued to F. K. Blades et al. on Sep. 10, 1991. Circuits employed in such instruments are disclosed in U.S. Pat. No. 4,683,435, issued to F. K. Blades on Jul. 28, 1987; U.S. Pat. No. 5,334,940, issued to F. K. Blades on Aug. 2, 1994; and U.S. Pat. No. 5,260,663, issued to F. K. Blades on Nov. 9, 1993.
As typically implemented in the cited patents, the instruments for measuring the total oxidizable carbon content of water disclosed in the cited patents comprise a sample cell wherein a static sample of water is maintained between a pair of conductivity-measuring electrodes while the sample and the electrodes are exposed to an irradiation source emitting UV light in the 184 and 253 nanometer wavelengths. The electrodes are typically formed of solid titanium oxidized to provide a TiO2 surface; this N-type semiconductor material catalyzes the reaction of organic carbon compounds in water to CO2 when exposed to short wavelength UV.
Operation of these instruments is subject to some uncertainty. Conductivityxe2x80x94the variable such instruments most immediately targetxe2x80x94fluctuates as a function of the temperature of the sample. This is a concern. The standard temperature at which conductivity values are typically reported is 25xc2x0 C. However, the sample liquid temperature is hardly ever exactly at 25xc2x0 C. when measured.
To account for such temperature effect, many basic conductivity measuring instruments attach or incorporate a thermal sensor to or into their outer cell walls. From such xe2x80x9cextracellularxe2x80x9d location, temperature readings are taken and imputed to represent the temperature within and throughout the cell""s internal volume. The variance from 25xc2x0 C. is then calculated into the measured conductivity to correct for any temperature-effect. Examples of such prior art instruments can be found in the patent literature.
For example, U.S. Pat. No. 5,047,212, issued to F. K. Blades et al. on Sep. 10, 1991, discloses a disk-shaped conductivity measuring instrument comprising a circular inner electrode and a concentric outer electrode. The circular inner electrode forms one face of the instrument""s internal enclosed volume, with the concentric outer electrode seated therein. A ring-shaped photo-oxidation source is incorporated into the instrument, proximate to the enclosed internal volume. A thermistor is integrated into the inner electrode and is used to measure the water temperature in the cell. In early derivatives of this technology, the inner electrodes tended to be comparatively bulky, and accordingly, had comparatively high thermal mass. Later derivativesxe2x80x94see e.g., the Access 643 TOC Analyzer available from the Anatel Corporation of Boulder, Colo.xe2x80x94offered cells with significantly reduced mass (e.g., circa 1.73 g).
As another example, U.S. Pat. No. 5,677,190, issued to P. C. Melanson et al. on Oct. 14, 1997, discloses xe2x80x9c[a]n improved measurement cell . . . for measuring the electrical characteristics of a liquid sample during exposure to radiation.xe2x80x9d More particularly, a glass cell forms a main tube extending generally parallel to an elongated photo-irradiation source. A pair of electrodes are disposed axially within the sample tube. Inlet and outlet tubes and a xe2x80x9ctemperature sensing wellxe2x80x9d are fused to the main tube.
Although these and other prior art conductivity measuring instruments continue to be useful, in respect of capturing very fine conductivity measurements, certain observations can be noted regarding their precision in the measurement of temperature.
First, in many prior art instruments, temperatures sensors are often secured to the housing of the cell, isolated from the liquid loaded within the cell reducing or otherwise frustrating thermal accuracy.
Second, the conduct of photo-oxidation on a sample liquid will in time produce thermal gradients within that sample. Thus, for example, the temperature of the sample liquid closest to the irradiation source will be higher than the temperature of the sample liquid furthest from the source. This can reduce the precision and/or accuracy of prior art instruments wherein the temperature sensor is remote from the electrodes. Such sensors will not measure the temperature in the area wherein conductivity is measured. Though one may wait for the photo-oxidized sample to equilibrate, this is often impractical.
Third, in prior art devices wherein the thermal sensor is embedded into a bulky electrode, the comparatively high thermal mass of such electrode will frustrate precise and/or accurate thermal readings. Depending on the materials used for such electrode, the temperature sensor may in operation be measuring the temperature of the electrode, rather than the sample liquid. This issue becomes compounded in instruments wherein the sensor is located within its own housing, surrounded by a variety of cell components, each such component having its own thermal mass and thermal conductivity relative to the liquid.
Fourth, in prior art instruments wherein the temperature sensor is located adjacent the outer wall of the cell, unmitigated thermal conduction through exterior walls can lead to an inaccurate temperature measurement. Depending on the materials used for the construction thereof, an instrument""s cell wall can conduct the temperature of the external ambient environment to the sample liquid in the vicinity of said cell wall.
In light of the above, there is a need for a new probe and cell design for use in a photo-oxidation based conductivity measuring device, which improves upon the present probes and cells currently used therein in respect of providing more precise temperature readings.
In consideration of the above need, the present invention provides a device capable of making an improved determination of the oxidizable carbon content of a liquid by obtaining a precise thermally-corrected conductivity measurement of a photo-oxidized sample of said liquid. The conductivity measurement isxe2x80x94as is the case in other prior art devicexe2x80x94thermally-corrected, but is particularly characterized by its unprecedented precision in the face of thermal gradients often produced during photo-oxidation. This precision is accomplished, in part, by the unprecedented incorporation into the device of certain materials and structures that permit the taking of xe2x80x9cintracellularxe2x80x9d thermal measurements (i.e., thermal measurements deep within the device""s internal volume) without undesirably compromising other important cell functions.
In one principal embodiment, the device comprises a cell, at least two elongate probes, and a temperature sensitive element (e.g., a thermistor). The cell comprises a rigid light-transmissive outer wall enclosing a continuous predetermined internal volume The elongate probesxe2x80x94which provide collectively the means to measure temperature and conductivityxe2x80x94penetrate through the rigid outer wall and extend substantially into the cell""s internal volume. At least one of the elongate probes is hollow at least partially along its length, with the temperature sensitive element positioned well within the resultant bore.
Two principal configurations are envisaged. In the first, two elongate probes are utilized, the pair functioning as electrodes for the measurement of conductivity, with one being hollow for the placement of the temperature sensitive element. In the second, the temperature sensitive element is placed in a hollow elongate probe positioned between paired elongate electrode probes.
In light of the above, it is a principal object of the present invention to provide an improved device capable of being used for photo-oxidizing a liquid and for obtaining a precise thermally-corrected measurement of the total oxidizable carbon (TOC) content thereof.
It is another object of the present invention to provide a methodology for determining the total oxidizable carbon (TOC) content of a liquid, the methodology involving a combination of both photo-oxidation steps and steps designed to reduce the effects of thermal gradients induced thereby.
Other objects will be apparent from the detailed description below and would include, for example, the provision of:
a conductivity probe having an improved thermal reading capability and greater sensitivity in conductivity readings especially at low conductivity levels;
a conductivity probe containing a thermal sensor having a thermal reading capability of +/xe2x88x921.0% as compared to the temperature of the liquid as measured by a salt thermometer test;
a conductivity probe containing a thermal sensor having a thermal reading capability of +/xe2x88x920.75% as compared to the temperature of the liquid as measured by a salt thermometer test;
a conductivity cell having a low cell constant and a conductivity probe having an improved thermal reading capability;
a conductivity cell having a cell constant less than approximately 1 cmxe2x88x921;
a conductivity cell having a cell constant between approximately 0.05 to approximately 1 cmxe2x88x921; and
a conductivity cell having a cell constant between proximately 0.1 and approximately 1 cmxe2x88x921 and capable of taking precise thermal measurements.
For further understanding of the nature and objects of the present invention, reference should be had to the following description considered in conjunction with the accompanying drawings.